Electrometer



Patented July 23,1940 y 2,209,138

ELECTROIVIETER Reinhold Rosenberger, Berlin-Friedenau,

f Germany Appucation, october 5, 1937, serial No. 167,469. Renewed May 3, 1940. In Germany October 9,

8 Claims. (Cl. 175-183) The presentinvention relates. to electrical ance attains a few hundred ohms. The appameasuring apparatus and more particularly perratus thus is useless for making measurements tains to the circuits of such apparatus. over a broad range without the risk of faulty Apparatus adapted to be used for measuring measurements caused by a reversal of the voltseveral different electrical constants of subage. `Errors of this kind are particularly dis- 5' stances such as gases, solutions, and solid bodies, turbingwhen employing such apparatus in and involving the use of thermionic valves are chemical laboratories for ascertaining voltages, known. Thus, in British Patents Nos. 309,038, such as polarization voltages, conductivities, par- 364,939 and 368,998 there is described a type of ticularly of liquids, and also of di-electrical con- 10 apparatus suitable for electrochemical measurestants of solid and liquid bodies. l0

ments, with the aid of which voltages, resist- It is an object of the present invention to proances, and capacities of substances, respectively, vide apparatus of the aforesaid kind in which can be measured by correspondingly switching the possibility of such uncontrollable errors of over to different circuits. In apparatus of. this indication is substantially suppressed or even type, there is provided an oscillation generating entirely eliminated. 15

circuit causing a high-frequency oscillation, In the annexed drawings Whose amplitude depends on the characteristics AFigures 1 and 2 are diagrammatical represenof the substances to be interposed in the grid tations of. curves taken from observations with circuit of the valve, which high-frequency oscilthe aid of the hitherto known type of apparatus,

lation is then employed in the measurement of whereas 20 the resistance or capacity of thesubstance to be Figures 3, 4 and' 5 are similar curves obtained tested. The characteristic values ascertained are with the apparatus according to the invention. preferably indicated on the dial of a measuring Figures 6, 7 and 8 are diagrammatically repreor indicator instrument provided in the anode sentative of the circuiting arrangement in accircuit of, the valve. l cordance with the present invention,I Figure 6 25 The generation of the oscillations in the referring to the arrangement to be used for known type of apparatus is generally veffected by measuring resistances, Figure 7, the same for an inductive back coupling between the anode measuring capacities, and Figure 8, the same for and the grid circuit. either of the two back measuring voltages.

coupling coils, being shunted by a non-variable Figure 9 is representative of the manner in 30 or variable condenser, which latter, together which the three types of. circuit arangements with the coil, determines the frequency of the represented in Figures 6, 7 and 8 may be emoscillation. bodied in a single apparatus.

Although apparatus of the aforesaid type have In Figure 1, the current values .i relating to been successfully employed in practice,` yet a apparatus of known type indicated on the dial 35 number of technical disadvantages are still -in of the instrument (solid line), and the variation herent therein: One of the main disadvantages of these current values with the value to be resides in the fact that the current occurring in 'measured are plotted against the resistance the indicator instrument varies with the value .values R to be measured. The curves show that 40 to be measured in a non-uniform, and of course a maximum in the current value occurs at about 40 in a non-linear manner. The effect of the prop- 500 ohms, which makes it clear that the known erty of the substance to be tested, and repretype of apparatus can only be employed for sented by the constant to be ascertained by measuring resistances either exclusively above, measurement, upon the amplitude of the oscillaor exclusively below, that value.

tion, and thus also upon the currentoccurring Figure 2 shows a similar curve obtained from 45 in the indicator instrumentinterposed in the the known type of apparatus, when measuring anode circuit, is generally such that even maxima capacities C: Also in that case the i-curve, and and minima occur when a steadily increasing still more the resistance is interposed in the grid circuit. The f Ai current occurring in the indicator instrument KC" may at rst increase in accordance with a certain rule and thereafter suddenly fall off. vIn are highly irregular. measuring resistances, the point at which such In contradistinction, similar curves represudden .decrease takes place, in theV known type j sented in Figures 3 and 4 and obtained with the 0f apparatus, generally OCQl-IIS when the resistaid of the improved apparatus according to the'55 'Curve 50 present invention show that in the measurement of resistances the z'curve is extremely uniform,

no maximum or minimumoccurrng up to about 8000 ohms, and that the corresponding well as the sensitivity of the measuring instrument are both highly dependent on the attenuation of the oscillating circuit. In the known typeof measuring instruments, the attenuation of the oscillating circuit was deliberately kept as low as possible in order to obtain the largest possible amplitude of the oscillations. In carrying out theI present invention certain definite damping resistances are interposed in the oscillating circuits eitherin the form of additional ohmic resistances or in the form of circuit ele.

ments having a high attenuating effect. Thus by shunting a resistance of s'everal thousand ohms/in Aparallel with the oscillation circuit coil and/the condenser,the sensitivity ofthe apparatus is considerably increased, Above and below the numerical value of 3000 ohms resistance, the sensitivity of the instrument appreciably decreases as shown in Figure 5, in which the logarithms of ,l m are plotted against the values of the shunted damping resistancel on the basis of an average measuring resistance of between about 1000 `to 1500 ohms. The total attenuation of the oscillating circuit, the coils of which are preferably wire coils, when expressed in the form of simpl by the attenuationde'crement e, amounts to about 3.4 and may vary between about 3.0 and 3.8. An attenuation of approximately the same order can be also obtained by interposing a resistance in series in the oscillating circuit.

It was further observed that when the capacity of the oscillating circuil is increased, the maxima of the i-curves shiftl to substantially higher R-values and vare vso etimes even completely euminated. Wenst nl the apparatus of the hitherto customary type theoscillating circuit condenser had a capafcity or between about 50 and 1000 cms., this capacity in accordance with the present inventionlis increased to approximately 4000 cms., particularly for the purpose oi' measuring the conductivity of liquids.

An improved uniforfmity of the i-curve and a fairly uniform and high sensitivity of the measuring instrument is obtainable also in measuring capacities by interposing, a resistance of several thousand ohms in series, with the oscillating circuit coil and the condenser having a capacity of the order-of 1000 cms.

Referring to Figure 6, the grid and the cathode rof the thermionic valve m are connected lwith the resistanceR to be measured. In the grid circuit there is included a coil g1 inductively coupled with a coil Z1 in the anode circuit. In parallel with coil l1 there is provided a non-variable condenser ci which, together with the coil li, determines the measuring oscillation,.which latter when measuring resistances is of4 the order of 10 kilocycles, preferably about 12 kilocycles. The anode circuit of the thermionic valve includes the indicator instrument i. In the known type of apparatus the condenser c1 disposed'in parallel With the fairly undamped coil li, had a capacity of not more than about 50 to 1000 cms. and the resistance ri shown in Figure 6 was absent altogether. In carrying the invention intoA practice the capacity value of the condenser ci is raised to about 4000 cms., and the resistance r1 of abouty 3000 ohms is shunted in parallel.

This arrangement thus enables the measuring of resistances ranging up to about 8000 and occasionally even to 10,000 ohms while avoiding,

within the range of measurement, the occurrence of maxima and/or unsteadiness in the indication.

When measuring capacities the grid circuit/ `coil of the thermionlc valve is connected with the somewhat higher damping resistance -r2 for the 'damping Aresistance r1 employed in Figure 6, r2 being about 5000 ohms.

When measuring pure voltages the thermionic valve no longer serves as an oscillation generator,

but simply as a thermionic valve voltmeter, -preferably in combination with a capacity circuit, in which the instrument i is disposed in the diagonal of a Wheatstone bridge, one member of which is lformed by the anode current space of the valve as shown in Fig. 8. The grid space of the valve is connected with the voltage S to be measured.

In all cases a monogrid valve having an anode tension of about 130 volts, an amplification factor of about 3.3% and a mutual A. C.conductance of about is preferably employed. But even when employing valves having characteristics different from the aforesaid ones, the advantages in accordance with the present invention are completely obtained.

In Figure 9 the thermionicvalve m, preferably a monogrid valve of types REN904 or A4110 Telefunken is operated from the mains, this being a further advantage over the type of apparatus hitherto employed, in which a battery was provided for this purpose. The anode voltage, after transformation, rectification and smoothing, is tapped from a voltage divider and conducted via resistances a, which are partly disconnectible, to the valve anode. The current leading to the indicator 'instrument i is also tapped from the voltage divider t by means of a tap e. The negative pole of the voltage divider t is, in the case of resistance measurements via anode coil Z1, and in the case of capacity measurements via anode coil l2, respectively connected with earth, the latter being also connected with the' cathode of valve m. The anode coil li is further inductively coupled with the grid circuit coils gi. The grid circuit coils, and if de- 'loi f denitely correct.

sired also the anode circuit coil are preferably made double in order to obtain the highest possible coupling factor, and thus also a large oscillation amplitude. It is also possible to increase the coupling factor, 'when using a simple backcoupling arrangement between only two coils, by disposing in the coils high frequency iron cores. In the case of capacity measurements the grid circuit coil g2, which is inductively back-coupled with the anode coil Zi, is in operation.

Each of the grid coils of the two oscillation circuits is connected with the grid of the valve m through a resistance v1 and o2, respectively. The ohmic values of these two resistances are different and the resistor v1 has only a few hundred ohms, such as 300 ohms, sothat the negative grid bias caused by the rectied oscillations in the grid circuit is thus relatively low. The resistor vz is of about 2000 ohms and thus causes a stronger negative grid bias on oscillation. A non-variable condenser p1, connected in parallel with the resistance v1, has a capacity of about 0.5 pf., while a condenser p2, connected in parallel with resistance v2, has a capacity of about 500 cms., considering that the frequency in this oscillating grid circuit is appreciably higher than in the grid circuit Zigi.

The substances to be tested are connected at I and II, respectively, with the grid of the valve preferably in such a manner that I is connected with the substance to be tested, while II is connected with a standard substance. It is also possible to dispense with a standard substance using a calibrated table showing the (standard) icurve. Since it has been observed that in spite of the improvements made in accordance with the present invention, the measurement of eX- tremely minute values, particularly when employing very small measurement resistances, still gives rise to the .occurrence of maxima or unsteadiness and indeniteness in the values taken from the indicator instrument, there is further provided a resistance r of approximately three hundred ohms in the lead connecting the substance to be tested to the cathode. 'By suitably selecting this resistance r, which is inter-1. posed in the grid circuit and always in series with the substance to be measured, the zero point of the range of measurement is shifted to a point beyond which the measurements are Since numerous tests have shown that the unsteadiness in the indicator current and the maxima of the latter invariably occur below about 300 ohms, it follows that the interposition of resistance 1' of 330 ohms results` in reliably preventing the occurrence of such unsteadiness. Between the resistance 1' and the cathode, there is further provided a condenser c of about 0.5 af. for the purpose of allowing only the alternating current used for measurement to pass through this resistance, while preventing such resistance r from giving rise to a direct voltage having a detrimental influence upon the grid-bias. Finally, resistance 1' is bridged by a non-Variable condenser k of about 2000 cms. so as to enable the oscillations toureach the grid unhampered by the substance to be measured when performing capacity measurements with the aid of oscillating circuit lz.

The different connections required for voltage, resistance and capacity measurements, respectively, are established by means of a series of switches. The switches I are to be closed in voltage measurements. The switches 2 are closed for resistance measurements and the switches 3 are closed for capacity measurements.v When measuring voltages the two oscillating circuits licz and lzcz, are completely disconnected, the circuits in operation thus corresponding to the arrangement shown in Figure 8. In this case, the anode circuit resistances r in Figure 9 and alsothe tap e on the voltage divider t are adjusted se that the necessary bridge equilibrium is established. When measuring resistances, the oscillation\generator circuit including the coil Z1, the condenser ci and the resistor n is put into operation together with the back-coupling coils g1 and the series resistance v1, while oscillating circuit formed of Vvthe coil l2 and the condenser c2 together with its series resistance v2 are disconnected. The reverse directions apply for capacity measurements.

Preferably, the coils employed in the two oscillating circuits are simple wire cylinder coils, the diameter of the wire being between about 0.1 to 0.4 mm., and the4 number of turns amounting to several hundreds.

The indicator instrument i is provided with one or more additional shunts in known manner.

What I claim is:

1. In apparatus for measuring electrical resistances and'capacities, a thermionic valve including a cathode and an anode and a grid-electrode,

a grid circuit and an anode circuit'comprisingl means for generating high-frequency-oscillations consisting of two oscillation circuits each containing an inductance coil and a capacity element, back coupling means between the anode and the grid circuit, a switch adapted to connect said valve to one of said oscillation circuits for the measuring of resistance and to the other circuit for the measuring of` capacity, means for connecting the electrical constant to be measured between said cathode and said grid, an indicating instrument in the anode circuit, and each of the high-frequency-oscillation circuits including a damping resistance of at least one thousand ohms disposed in parallel to the inductance coil and the capacity element of said circuit.

2. In apparatus for measuring electrical resistances and capacities, a thermionic valve including a cathode and an anode and a grid-electrode, a grid circuit and an anode circuit comprising means for generating high-frequency-oscillations consisting of two oscillation circuits each` 'l and the capacity element of said circuit, andthe total attenuation of said oscillation circuit having a logarithmic decrementof between 3.0 and about 3.8.

3. In apparatus for measuring electrical resist- .ances and capacities, a thermionic valve including a cathode and an anode and a grid-electrode,

means for generating high-frequency-oscillations consisting of two oscillation circuits each containing an inductance element, a capacity element, back coupling means between the anode .a grid circuit and an anode circuit comprising and thev grid circuit, a switch adapted, to connect said valveto one'ofsaid oscillation circuits for Vthe measuring of resistance and to the other high-frequency-oscillation circuits including aV damping resistance of atleast one thousand ohms in parallel to the lnductance element andthe thegrid circuit beingv inductive v coefficient of about 90%." Y 4. In apparatus for measuring electrical resist- A"ances and capacities, a thermionic valve having circuitY for the measuring of capacity, means for f connecting the electrical constant to be measured between said cathode and said grid, an indicating instrument in the anode circuit, each of the capacity element of said circuit, the inductive elements of saidwoscillation circuits consisting of simple lwire coils, the capacity element having a -capacity yof about 4000 cm. for the resistance' measuring circuit and a capacity of about 800` cm.

' for 4the capacity measuring circuit, the damping resistance being of about three thousand ohms for the resistance measuring circuit and of about 'five thousand ohms "for the capacity measuring circuit, and the coupling between the anode and and having a a'cathode and ananode and a grid-electrode, a grid .circuit and anfanode circuit comprising i means for Vgenerating'high-frequency-oscillations consisting'oftwo oscillation circuits each'con- Ataining an inductance coil and a condenser, back coupling means between the anode and the .grid circuit, a switch adapted to connect said valve to one'of said oscillation lcircuits forl the measuring of lresistar'nfzre Vand to` the' other circuit for `the -measuring oif'cap'acity, means for connectingthe V.electrical constant to be measured between said cathode and said grid, an indicating instrument "in the anode circuit, each of the high-frequencyfoscillation circuits including a damping resistance of at leastone thousand ohms-disposed in parallel to the inductance and the capacity of said circuit, the'condenser having a capacity of aboutf4000'cm. for the resistance measuring cir- "cuit-and a capacity'of about 800 cm. for the capacity measuring circuit, the damping resistance being of about threethousand ohms for the resistance ,measuring circuit and of about iive thousand'ohms for the capacity measuring circuit, the coupling between the anode and the grid circuit being inductive and having a coecient of about v90%, andiat. least one of the backcoupled coil systems consisting of two single coils disposed inparallel.k

fg 5. In apparatus for measuring electrical resistances andcapacities, a ,thermionic valve containin'g'a cathode and an anode and a grid-electrode,

.a grid `circuit and an anode circuit comprising means for generating high-frequency-oscillations consisting Yof two Voscillation circuits each containing an'inductance coil and a condenser, back coupling means between the anode and the grid circuit, a switchA adaptedrto connect said valve to one of said oscillation circuits for the measurv-ing 'of resistance and to the other circuit for the measuring of capacity, means for connecting the electrical constant to be'measured between said Vcathode and said grid', a grid bias resistance ofat least about'one hundred ohms disposed in series between said electrical constantV and said cathode, an indicating instrument in the anode cir--v cuit, leachl of the high-frequency-oscillation cir- 'cuits including a damping resistance of 'at least one thousand ohms'disposed in parallel to the induct'ance coil and the condenser of said circuit.

6. In apparatus for measuring electrical resistances and capacities, a thermionicA valve containing a cathode and an anode, 4and aV Vgridelectrode, a grid circuit and an anode circuit com-` prising means for generating high-frequency-oscillations vconsisting of two oscillation vcircuits each containing an inductance coil land a condenser, Vbaci-1 couplingmeans between vthe anode and the Vgrid circuit, a switch' adapted to connect said valve to one of said'oscillation circuits for.

the 'measuring of resistance andV to the other circuit for the measuring of capacity, means for connecting the electrical constant to be measured between said cathode and said grid, a grid bias resistance offat least about one hundred ohms shunted by acondenserv of about 2000 cm. dis-V v a grid circuit and an anode circuit comprising means for generating5high-frequency-oscillations consisting of two oscillation'circuits each containing an inductance coil and a condenser, back coupling means between the anode and the grid circuit, a switch adapted to connect said valve to measuring of capacity, means forconnecting the electrical constant to be measured between said cathode and said grid, Yan indicating instrument in the anode circuit, one of the terminals of said instrument being connected overa resistance to the positive side of a potential source and to the anode of the valve, the other terminal being connected over a resistance to the positive potential and'over a. resistance tothe oscillation circuit, and each of the high-frequency-oscillation circuits including a damping resistance of at least Vone of said oscillation circuits for the measuring of resistance and to the other circuit for the one thousand ohms being disposed in parallel to the inductance coil and` the condenser of "said circuit.' -Y

8. In apparatus Vfor measuring electrical resistances and capacities, a thermionic valve containing a cathode and an anode and a grid-electrode,` a grid circuit and an anode circuit comprising means for generating high-frequency-oscillations consisting of two oscillation circuits each containing an lnductance coil and a condenser, back coupling means between the anode and the grid circuit, a switch adapted tu vconnect said valve to one of said oscillation circuits forthe measuring of resistance and to the other 'circuit fory theY measuring of capacity, means for connecting the Y electrical constantV to be measured between said cathode and said, grid, an indicating instrument in the anode circuit, one of the terminals of said instrument being connected over a 'resistance to the positive side of a potential sourceV and to the anode of the valve, the other terminal being connected over a resistance to the positive potential and over a resistance to the oscillation circuit, Y switches provided for disconnecting said oscillation circuit for the purpose of measuring potentials, said resistances connected to the terminals of said instrument then forming Ithree members of a Wheatstone bridge the fourth member of Vwhich is formed by said thermionic valve and one of the diagonals of which ,beingV formed by the instrument and the other diagonal by the anode potential source. f Y

REINI-IOLD ROSENBERGER. 

